The present invention relates to an absorption chiller-heater and a method for forming an initial anticorrosive film therefor, and more specifically to an absorption chiller-heater in which a very good initial anticorrosive film is formed by using a special absorbent solution for the absorption chiller-heater and a special steel parts surface on which an anticorrosive film is to be formed as well as a method for forming such an initial anticorrosive film.
Generally, absorption chiller-heaters (absorption refrigerating machines) use an absorbent solution based on lithium bromide, and water as a refrigerant. In the present specification, the term "absorption chiller-heater" includes an absorption chiller which only functions to chill cold water as well as an absorption chiller/heater which may function as a chiller and also as a heater to heat hot water. The high-temperature regenerator used therein constitutes a heat engine. Therefore, in order to increase efficiency within a practical range, a double-effect refrigerating system may be used, so that the temperature and concentration of the absorbent solution in the high-temperature regenerator reach about 160.degree. C. and 65 wt %, respectively. The absorbent solution becomes more corrosive for materials such as steel, copper, CuNi, as the temperature and concentration rise.
Well-known corrosion inhibitors mainly directed to steel materials include chromates, nitrates and molybdates. It is generally known that these corrosion inhibitors are added to an absorbent solution together with an alkali metal hydroxide to form an anticorrosive film on the material surface of an absorption chiller-heater once operated.
However, chromates have the problem of pitting due to their excessively strong oxidative power, and nitrates also have the problem of pitting and corrosion of copper materials. Molybdates have little danger of pitting because their oxidative power is too weak to raise corrosion potential in steel materials to a level where pitting would occur. However, an absorbent solution containing a molybdate corrosion inhibitor alone or an anticorrosive film-forming method according to the prior art does not form a sufficiently good anticorrosive film and may cause a problem of deterioration in refrigerating capacity under the influence of hydrogen gas generated by continuous corrosion reaction, because the molybdate may be dissolved in the lithium bromide absorbent solution only at a low level.
Recently, mixed-type corrosion inhibitors based on molybdates have also been developed and employed. Corrosion inhibitors to be mixed with molybdates include chromates, nitrates, borates, etc. However, such mixed-type corrosion inhibitors are not superior in anticorrosive power to molybdates alone, but involve the problem of pitting or crevice corrosion.
According to the prior art, a good anticorrosive film could not be formed in an assembled absorption chiller-heater using a corrosion inhibitor comprising a molybdate alone. Thus, the anticorrosive film formed was incomplete and a large amount of hydrogen gas is generated by corrosion reaction. If the amount of hydrogen gas generated exceeds the capacity of the bleeder of an absorption chiller-heater, the hydrogen gas will remain as non-condensed gas in the chiller-heater to inhibit absorption or evaporation action, thus causing a deterioration in refrigerating capacity. Even if the capacity of the bleeder were enhanced, the large amount of hydrogen gas would be generated by corrosion reaction, and would produce corrosion products (sludge) according to the amount of hydrogen gas. When accumulation of sludge in the chiller-heater becomes high, it blocks circulation of the absorbent solution in a solution heat exchanger or the like to hinder operation of the absorption chiller-heater.
It has been recognized in the prior art that molybdates are better corrosion inhibitors free from pitting or crevice corrosion as compared with chromates and nitrates. However, the problem of evolution of a large amount of hydrogen gas or the like has not been solved, because the disadvantage of the low level of molybdates which may be dissolved in absorbent solutions was not sufficiently improved or investigated to perfectly protect an assembled absorption chiller-heater with molybdates against corrosion. Global considerations including absorbent solution formula, steel parts surface on which an anticorrosive film is to be formed, initial anticorrosive film-forming method and other factors would naturally be required to form a good anticorrosive film in an absorption chiller-heater, but no such comprehensive solutions have been considered to date.
There are two reasons why a good anticorrosive film can not be formed with molybdates in an assembled absorption chiller-heater in the prior art. One is that the concentration of molybdates in absorbent solutions was lower than the required level during the initial anticorrosive film-forming period. The second reason is that the surface state of the steel parts on which an anticorrosive film is to be formed was not considered despite of a low oxidative power of molybdates.
Especially, the surface state of the steel parts on which an anticorrosive film is to be formed should be optimally prepared for molybdates, because the surface state influences not only the amount of hydrogen gas generated during the initial anticorrosive film-forming period but also the amount of hydrogen gas generated per unit time during later stages in an absorption chiller-heater. As used herein, the term "initial anticorrosive film-forming period" means a period from the start of formation of an anticorrosive film to the time at which the amount of hydrogen gas generated per unit time exponentially decreases to reach an almost stable level.
The reason why the above two problems have not been satisfactorily solved seems to exist in rare consciousness of those problems because previously common corrosion inhibitors such as chromates or nitrates have high solubility in absorbent solutions and a high oxidative power.
In view of the above prior art, it is an object of the present invention to explain the relation of three conditions consisting of absorbent solution formula conditions, steel parts surface conditions and initial film-forming conditions by performing various basic corrosion tests and various practical corrosion tests using a molybdate as a single corrosion inhibitor, to select optimal combination of the conditions of absorbent solution, steel parts surface on which an anticorrosive film is to be formed and operation for forming a good initial anticorrosive film in an assembled absorption chiller-heater, and thereby to provide an absorption chiller-heater in which an anticorrosive film with a very high anticorrosive power is formed and a method for forming such an initial anticorrosive film.